Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor, a condenser, an expansion device, e.g. in the form of an expansion valve, and an evaporator arranged along a refrigerant path. Refrigerant circulates the refrigerant path and is alternatingly compressed and expanded. Heat exchange takes place in the condenser and the evaporator, and it is thereby possible to provide cooling or heating to a closed volume, e.g. a room or a refrigerated compartment or box.
In the case that the expansion device is in the form of an expansion valve, the expansion valve expands the refrigerant and controls the supply of refrigerant to the evaporator. The amount of refrigerant supplied to the evaporator is determined by the opening degree of the expansion valve.
To this end a fixed orifice expansion valve may be used. A fixed orifice expansion valve supplies refrigerant to the evaporator via an orifice with a fixed size. This is very simple, and the valve is easy and cost effective to manufacture. However, it is not possible to adjust the supply of refrigerant to the evaporator, e.g. in response to a measured value of the superheat of refrigerant leaving the evaporator, because it is not possible to adjust the opening degree.
In vapour compression systems where it is necessary or desirable to be able to control the refrigerant supply to the evaporator, a thermostatic expansion valve may be used. The opening degree of a thermostatic expansion valve is adjustable, and an opening degree which accurately results in a desired superheat value can therefore be selected at any given time. However, thermostatic expansion valves are relatively expensive, and they are therefore normally not applied in low cost vapour compression systems.